1. Field of the Invention
The present invention relates to a protective film for protecting a dielectric layer of a plasma display panel from discharge, a method of forming the same, a plasma display panel and a method of manufacturing the same, and more particularly, to a protective film of which a discharge characteristic is improved, a method of forming the same, a plasma display panel and a method of manufacturing the same.
2. Description of the Related Art
Generally, a plasma display panel (PDP) has a thin structure, no flicker and a great display contrast ratio. Also, the PDP has a large number of features that it can be manufactured as a relatively large screen, its response speed is fast, a multicolor light emission is possible by using a fluorescent material because it is a spontaneous light emission type, and the like. Therefore, recently, the PDP has been widely used for the display device field related to the computer and the color image display field.
In the plasma display according to an operating method, there are AC driving type where electrodes are coated with a dielectric and operated indirectly in AC discharge state and DC driving type where electrodes are exposed to a discharge space and operated in DC discharge state. Further, in the AC driving type plasma display, there are a memory operating type to use memory of discharge cell as a driving method and a refresh operating type not to use it. And, brightness of the AC driving type plasma display is proportional to the number of discharge times. In case of the above refresh operating type, it is mainly used in the plasma display having small display capacity because brightness decreases when display capacity increases.
FIG. 1 is an exploded view schematically showing a structure of the AC driving memory operating type color plasma display.
In PDP, two isolation substrates 101 and 102 made of glass are provided. The isolation substrate 101 becomes a rear substrate, and the isolation substrate 102 becomes a front substrate.
On the isolation substrate 102, transparent electrodes 103 and 104 are provided on a face side opposite to the isolation substrate 101. The transparent electrodes 103 and 104 are extended in a horizontal direction (transverse direction) of the panel. Also, trace electrodes 105 and 106 are arranged to overlap the transparent electrodes 103 and 104, respectively. The trace electrodes 105 and 106 are made of, for example, metal material and provided in order to lower electrode resistance value between each electrode and external driving device. Further, there are formed a dielectric layer 112 covering the transparent electrodes 103 and 104, a plurality of black stripe layers 108 formed on the dielectric layer 112 and extended in a vertical direction (longitudinal direction) of the panel, color filter layers 110R, 110G and 110B of red color R, green color G and blue color B formed between the black stripe layers 108, and a protective film 114 for protecting the dielectric layer 112 and the transparent electrode 103 from discharge.
Also, because PDP emits each visible light of R, G and B by exciting the fluorescent material with emitted ultraviolet light, the color filter layers are not necessarily needed. The color filter layers are to collect spectrum of luminescent colors by the fluorescent material.
On the isolation substrate 101, data electrodes 107 perpendicular to the transparent electrodes 103 and 104 are provided on a face side opposite to the isolation substrate 102. Therefore, the data electrodes 107 are extended in the vertical direction. Further, a partition wall 109 is provided to divide a display cell in the horizontal direction. The partition wall 109 is opposite to the black stripe layers 108. Further, a dielectric layer 113 covering the data electrodes 107 is formed, and a fluorescent layer 111 to transform ultraviolet light generated by discharging of discharge gas into visible light is formed on a side surface of the partition wall 109 and a surface of the dielectric layer 113. Further, a discharge gas space is secured by means of the partition wall 109 in the space between the isolation substrates 101 and 102, and the discharge gas space is filled with a discharge gas consisting of helium, neon, xenon or mixture of gases thereof.
The protective film 114 is provided in order to protect the dielectric layer 112, the transparent electrode 103 and the like from sputtering by ion bombardment during discharge as mentioned above, and because the protective film 114 comes in contact with the discharge gas space, its material and film quality affect greatly the discharge characteristic. Further, in AC driving type PDP, low consuming power, simplification of driving circuit, high precision and larger screen are important factors.
Therefore, generally, magnesium oxide MgO is used as a material of the protective film 114. MgO is an insulator having excellent sputtering resistance and a large secondary electron emission coefficient. The driving of PDP becomes possible with lowering discharge starting voltage by using MgO.
Subsequently, a conventional method of forming the protective film in the PDP will be described. The protective film is generally formed by a vacuum deposition method. FIG. 2 is a schematic diagram showing a conventional film forming apparatus of the protective film.
In the conventional film forming apparatus, a deposition chamber 121 is provided. In an upper part of the deposition chamber 121, a substrate 124 in which a dielectric layer, etc. have already been formed and MgO film is formed is mounted. Also, in lower part of the deposition chamber 121, a deposition source 125 composed of MgO as a raw material of the protective film is mounted. Further, in the deposition chamber 121, a heater 132 heating the substrate 124 and a gas inlet (not shown) for O2 gas are formed.
In case of manufacturing the protective film using the conventional film forming apparatus configured as mentioned above, first, the substrate 124 is fixed in the upper part of the deposition chamber 121, and the substrate 124 is heated by the heater 132, and simultaneously, the deposition chamber 121 is exhausted. Subsequently, in order to arrange crystal orientation of MgO film, while oxygen gas is introduced into the deposition chamber 121, an electron beam 133 is irradiated to the deposition source 125 so that the MgO film is formed as the protective film on the opposite side to the deposition source 125 of the substrate 124.
Further, in order to improving an orientation property of the MgO film, the method of forming the MgO film in an atmosphere including hydrogen atom in excited or ionized state is disclosed (Japanese Patent Laid-Open No. Hei 9-295894 Publication).
Further, in order to lower a discharge voltage by improving the secondary electron emission coefficient of the protective film, the PDP in which an orientation of the protective film is in (n00) or (mm0) orientation and a surface roughness is 30 nm or more is disclosed (Japanese Patent Laid-Open No. Hei 11-3665 Publication).
However, in the display operation of the conventional AC memory operating type PDP, first, a discharge is generated in a discharge space by applying a discharge voltage pulse to the transparent electrodes 103 and 104. By this discharge, on the surface of the discharge space side of the protective film 114, a charge having opposite polarity to the polarities applied to each electrode is accumulated at the position where the transparent electrodes 103 and 104 face each other (wall charge forming step).
Then, a discharge is generated once more in the discharge space by applying a voltage having opposite polarity to the above discharge voltage pulse to the transparent electrodes 103 and 104. The accumulated charge (wall charge) is erased by this discharge so that the wall charge does not exist in the entire surface of PDP (erasing step of wall charge or erasing step).
Subsequently, the transparent electrode 103 is scanned by applying a predetermined voltage in turn, and a wall charge is accumulated as a preparation for displaying a light emitting cell by applying a predetermined voltage between the transparent electrode 103 in the voltage applying state and the data electrode 107 corresponding to a light emitting cell to be displayed out of the light emitting cells belonging to the transparent electrode 103 (writing step).
Next, an image display is performed by applying a sustaining discharge pulse voltage to the transparent electrodes 103 and 104 on the entire surface of PDP. And, because the voltage value of the sustaining discharge pulse voltage is set to be lower than that of discharge pulse voltage light emission is not generated, light emission does not occur in a light emitting cell in which the wall charge is not formed in the writing step, and light emission occurs only in a light emitting cell in which writing discharge is performed so that the image display is performed (display discharge step). In a gradation display, about 256 level display gradations are accomplished by combining in time series about eight kinds of sustaining discharge pulse groups of which the number of pulses is different according to the number of gradations (subfield gradation method).
FIGS. 3A and 3B are graphs showing the relationship between the applying voltage and discharge delay light emission in which an abscissa indicates the time and an ordinate indicates the light emission intensity and voltage. In the PDP, in case where there is no discharge delay, namely, light emission delay, because a discharge is started almost simultaneously in response to writing pulse applying start as shown in FIG. 3A, the light emission intensity characteristic having a very sharp peak is obtained. However, in case where there is discharge delay in each light emitting cell according to the secondary electron emission efficiency, each light emitting cell starts the discharge individually in response to the writing pulse applying start. Therefore, the peak of light emission intensity is lower and its width becomes wider than comparing with the case where there is no discharge delay, as shown in FIG. 3B. Further, all the light emitting cells do not start discharge simultaneously within the writing pulse applying time. Therefore, the light emitting cells in which writing is not still completed remain at the point of the writing pulse applying expiration time. Further, a portion indicated by a broken line in FIG. 3B shows an example of light emission intensity in case where the writing pulse is applied longer than the shown writing pulse period, and shows that the light emitting cells to be discharged cannot be discharged during applying the writing pulse, namely, the writing operating becomes incomplete. In this case, when the discharge delay is observed as light emission of the light emitting cell in the entire surface of the PDP, it is observed as flicker of screen display. Therefore, in case where the secondary electron emission efficiency of the protective film 114 (MgO film) is degraded, as shown in FIG. 3B, because the writing pulse applying time is shortened according to high precision and high gradation of the PDP, there is a problem that the discharge delay and writing operation becomes incomplete.
However, when the protective film formed by the conventional film forming method using the film forming apparatus shown in FIG. 2 is used, the forming time of writing discharge becomes longer so that a writing error, in which the discharge is not started within the defined times, occurs easily. Particularly, the discharge delay is long in the display cell, which becomes an isolation point in time and space, and in this case, a writing scan pulse width needs to be set longer. However, when the scan pulse width is set longer, there are problems that the number of sustaining pulses required for improving brightness is restricted and the driving by a dual scan to scan upper half part and lower half part of a screen individually is needed. In case of the dual scan, since the number of driving circuits is numerous comparing with a single scan, it is an obstacle to cut down cost. Further, since a crystal grain diameter is small in the conventional film forming method, there is a problem that the discharge starting voltage is high.
Further, in the film forming method described in Japanese Patent Laid-Open No. Hei 9-295894 Publication, the orientation property is improved, but orientation plane is not uniform. Therefore, there are some cases where sputtering resistance becomes insufficient. Further, the crystal grain diameter becomes small, and the discharge starting voltage becomes high. Similarly, in the PDP described in Japanese Patent Laid-Open No. Hei 11-3665 Publication, the sputtering resistance as a protective film is insufficient.